Signaling with high frequency waves



July 21, 1936. R K POTTER l Y 2,048,080

SIGNALING WITH HIGH FREQUENCY WAVES Filed Feb. l0, 1934 2 Sheets-Sheet ll\ e 4--E Q I/:Ly

ATTORNEY July 21 1935- R. K. POTTER SIGNALING' WITH HIGH FREQUENCYWAVES' 2 Sheets-Sheet 2 Filed Feb. lO, 1954 @WI/PML ATTORNEY PatentedJuly 21, 1936 UNITED STATES PATENT Aorifice SIGNALING WITH HIGHFREQUENCY WAVES Application-February 10, 1934, :Serial No. .710,707

14 Claims.

This invention relates'to methods of andmeans for signaling vWhichinvolve the transmission of the'signalby themodulation ofa'high-frequency carrier wave. More particularly the inventionrelates-to carrier transmission ,in which the carrier and a singleside-band are transmitted, the other side-band ibeing suppressed at thetransmitting endof `thesignaling system.

'-While the Vinvention is generally applicable to lO signaling withhigh-frequency waves in the manner rindicated hereinabove, whether thetransmitting'medium be the ether ora circuit formed by one or moremetallic conductors or a combination of one ormore radio linksandWirelinks,

15 itis :particularly applicable to radio broadcast transmission andwill bespecically described and discussed Vhereinafter with reference tosuch transmission.

Y .It is well-known in the art ofradio transmission that .it isdesirable that the range of frequencies necessary to transmission fromagiven transmitting station be reduced to limits as narroW1as dueregardior the quality of the signal transmission vWill permit.Accordingly, in the case .of radio broadcasting, in which it is 4usuallynecessary to transmit the carrierfrequency along withthe'sidefrequencies produced by modulation of @the carrier in accordance withthe signal, it has been proposed that one ofthe sidebands be 10removed-'by suppression at the transmitting station. vOne objection tothis proposal has been that the removal oi one of the side-.bands leadsto 'substantialndistortiom as will be discussed at greater lengthhereinafter. UU The :ultimateobject of the present invention istoreduce'the range of .frequencies which it is necessary totransmitWith'unsuppressed carrier transmission in order to produce high quality,and thus to permit a more eicient channel spacing :in such vtransmissionas radio broadcasting; the proximate objectl ofthe invention is toeliminate for practical purposes, or, at the least, very greatly reducethe distortion normally resulting from the suppression of oneside-bandin unsuppressed carrier transmission.

lngeneral, theabove-stated proximate object is accomplished byproducingat the transmitting stationan. auxiliary Wave and modulating the carrierlWaveby the auxiliary Wave, this auxiliary Wavehavingfa form such thatthere Will be efl fecteda substantial compensation for the distortionresultingfrom the suppression of the sideband. More specically, theproximate object mayibeaccomplished, after the suppression of 55one-ofthe side-bands, by derivingat thetransmitting station 'from .aportion of the transmitted energy a ,demodulated signal wave, whichwillrbe a distorted Vrepresentation of the signal input =wave,-eiecting-adjustment of the amplitude and phase :of :this 'demodulatedsignal wave, and 5 combining the demodulated signal wave with the signalinput wave for modulation with the carrier wave, ftheadjustment of thedemodulated signal -wavebeingsuch as substantially to compensate forthefdistortion introduced as a result 10 of the suppression lof the one-side-band.

,The rfollowing `description of the invention is to be :readwithreference to the accompanying drawings, of .which- Figure "1, 4by meansof `vector diagrams and 15 curves, illustrateslthe problemintroduced bythe suppression of one side-band in unsuppressed carrier transmission;

-Fig.v2, -by `means-of curves, indicates graphically the :nature of thecorrection provided by 20 the applicant; :and

Fig. 3 shows .diagrammatically, and in part schematically, -a lsuitablearrangement of circuits for effecting .the applicants elimination ormaterial ireduction of the distortion introduced 25 by thesingleside-.band suppression.

With reference rst to Fig. 1 of the drawings, it 'to be understood thatin the case of transmission-With the carriersuppressed at thetransmitting end of the systemand reapplied for demodulation at thereceiving end, the ratio of carrieramplitude to side-,band amplitude isreadily adjustable and may be madehigh so that any distortion.introduced yas a `rresult of the suppression oi oneside-band at'thetransmitting end 35 is negligible, v.Whereas in the `caseof thetransmission .of VVthe .carrier With one side-band, -with whichtheapplicantisdealing, the carrier amplitude `Will `exceed-the side-bandamplitude by a verytmuchvsmaller amountif the same effective level ofsignal isto be maintained atthe receiver, andthe result.of.;thesuppression of the one sideband :will be :distortion which is quiteconsiderable, asshowninthis ligure. The-vector diagrams and curves.arendrawn to scale and represent a particular 4case cof '15% modulationof a highfrequency zwave by'a single tone, it-being understood that thesingle tonecase does not diier in general principle from the case ofmodulation by azvoice wave, -.for instance, `but permits of a 50 moregraphic illustration.

`The upperzrow of diagrams of Fig. 1 represents inpterms ,of vectors 1azhigh-frequency wave modulated bythe :tonein `.the ordinary manner andcomprising .the :carrier (C) andthe two side- Y of carrier and twoside-bands, the amplitude vav angles with the carrier vector at anytime, the.V

direction of the resultantvector-is always that of the carrier vector.Accordingly, in ,thisY case riation represented by the resultant vectors(R1)` gives a modulated wave envelope corresponding to the modulatingwave (tone). spondence isY graphically shown by thediagram appearing atthe bottom` ofv Fig. 1, in which' the unbroken curve W1 is determined byplotting the amplitudes 'represented by the vectors R1, theenvelopeamplitude varying'with time in a simple sinusoidal fashion." Thus,inVthe case of carrier and double side-band transmission, the

, shape of the low-frequency signal wave' resulting fromproperdemodulation `at'the receiving end corresponds to the shape yofthe low-frequency signal wave applied at the transmitting'end. Y

If, however, after the modulation of the Vhighfrequency wave, one Vofthe side-bands issuppressed, the resultant-amplitude variation (or waveenvelope) no longer corresponds to the shape of the modulating Wave. `Inthe lower row of vectordiagramsV of Fig. 1,` the unbroken,sin gle-headedvectors C and S-represent the carrier and the unsuppressed side-band(or` sidefrequency), respectivelyi It will be noted that in order tomaintain the normal percentage amplitude modulation (75%) ortV thesamesignal level at a receiving pointftheamplitude of theunsuppresse-d-side-band S `is made tWice'as-fgreat asv the amplitude oftheside-band S1 or the sideband S2 in the case of carrier and doublesideband transmissiondiscussed hereinabove.V Since the-side-bandvector-S is now rotating with respect to the carrier Vector `C lwithoutthe directionally compensating effect of a side-band vector rotating inthe opposite direction, the resultant vectors no longer coincidedirectionally With the carrier vector, except at the beginning and endof each half-cycle, and the resultant amplitudes representedatjintervals-of 40 by the lengths of the broke-n; double-headed vectorsR2, are equal only at'those points to the resultant amplitudes at thecorresponding points in the case of the carrierand double side-bandtransmission. Accordingly, with thelsuppression of the one side-band,Athe amplitude variation represented by the vectors R2 will give amodulated-'wave envelope which does notv` correspond to themodulatingxwave` (tone). vThe distortion is graphically shown inthediagram at `the bottomV of Fig. l, in Vwhich theYA broken curve W2 isdetermined-by plotting the amplitudes represented by the vectors R2.It'will be noted that the envelope amplitudeY Varies with time in anon-sinusoidal fashion, the ldegree of distortion being indicatedbythedisplacement ofi curve W2 with respect tocurve W1.It'will'nowreadily be seen than-inthe case of carrier and singlesideband transmission, Vone side-band'- having been suppressed, theshape of thelow-freq'uency` signal-wave resulting fromdemodulationat.the

This corre- Y receiving end ofthe system Vis the resultof a receivingend does not, without the introduction of some correction, correspond toorvery closely approximate the shape of the low-frequency signal waveapplied at the transmitting end. In other words the signal is seriouslydistorted. V

It can now be seen that, in the case of Vcarrier and single side-bandtransmission, if theV distortion resulting from the'suppression of theone side-band is to be eliminated or very'greatly reduced, there must beapplied'torthe modul lator at the transmitting end, for modulation ofAthe carrier wave, a wave of such shape that after themodulation and thesuppression of the l side-band there can be derived at the receivingVend a. low-frequencyvwavesubstantially corre- 1 spondingv to theenvelope shape of the modulated altered Yin shape to correspondsubstantially to the Wave W1. l Y 2 In V'order to present this pointygraphically',v

there Vare shown in Fig.. 2 the unbrokenwaveV side-band case (these twowaves being the same as the waves of like designation in Figjl)V and-Vin addition a' wave W3 (shown in mixedr curve),

which is the auxiliary or corrected wave whichV must be used at thetransmitting end'toimodulate the carrier wave. of course, that Vthe waveWsof Fig. Z'represents the requirement for perfect correction; linprac-` tice, 'this wave will closely! approximate .the'form shown, withthe object of substantial elimination or very great reduction of thedistortionwhich would' otherwise resultfrom the side-band suppression.It can be seen from an examination of Fig. 2 that if the envelope shapeof the. wavezfrom which the signal. output is to be derived'at the Ycombination of a wave lhaving the shape of .curve W2 and a wavesubstantially corresponding to curve W3, there can be produced at, thereceiving end a low-frequency wavehaving substantially the shape Vof thecurve W1. It will be understood, however, that the wave representedv byW2 is .formed only as a result of the suppression of the one side-band.'Therefore, the problem must be solved by applying at the transmittingend for'modulation with the carrier a wave substantially in the'form ofW3; in other words, the final modulating energy acting on the carrier inthe modulator at the. transmitting station must be substantially'.represented by the curve W3. represent a distortion which .willbe'substantially. eliminated or olset by thedistortion subsequentlyintroduced as Va result of the side-band suppression. Viewed somewhatdifferently, the case is one in which there is introducedat theVtransmitting end an auxiliary distortion which kwill in Vthe ultimateeffect substantiallycompensate for the Vdistortion Vintroduced bytheside-,band suppression. Y Y

An arrangement of apparatus suitable for the practice of the inventionis shown in the circuit diagram of Fig. 3. At the transmitting stationthe signal energy from the audio-frequency input is passed through thetransformer T1, the audiofrequency amplier AF and the audio-frequencylter F1 to the vacuum'tube circuits of the modulator, in which thislow-frequency Wave modu- It will be understood,

The product of the-modulation will then lates the high frequency waveproduced' inthev carrier frequency oscillator, associated Withtheproduct of the modulation comprises. the carrier and the doubleside-band components. The output of the modulator is then passed througha high-frequency amplifier and through a lter F2, which suppresses oneof the two side-bands and passes the carrier and the other side-band.The carrier and single side-band energy is amplied in another.high-frequency amplifier and is radiated from the transmitting antennaA1. .Without correction in accordance with the invention, this energy,when picked up by the antenna A2 at the receiving station anddemodulated, would give the distortion indicated in Fig. l, whichdistortion has been shown hereinabove to be the result of thesuppression of the side-band, as in filter F2.

For the purpose of effecting the desired correction in accordance withthe particular arrangement of Fig. 3, a portion of the high-frequencyenergy radiated from the antenna A1 is picked up by the auxiliaryreceiving antenna A3 at the transmitting station. In this auxiliarycircuit a demodulator D, which is of the type preferably used at thereceiving station, serves to demodulate the receivedportion of thesignal energy, and the output of the demodulator is then passed throughan audio-frequency amplifier. By means of a phase-correcting network N1and an attenuation network Nz, elements which in themselves are wellunderstood in the art, the demodulated signal is adjusted in phase andamplitude and is then impressed on the input or grid circuit of theaudio-frequency amplier AF in the main transmission circuit, through thetransformer T3. It has been seen that the signal inputis normallyVimpressed on this grid circuit of tube AF through transformer T1; it isnow seen that with the circuit arrangement of Fig. 3, a combination ofthe normal input signal wave and the auxiliary demodulated signal wave,the latter having been subjected to suitable adjustment, can beimpressed on the amplier AF and passed through the audio-frequency lterF1 for modulation of the carrier wave. The fundamental component fromthe auxiliary circuit should be adjusted in networks N1 and N2 so thatit will be out of phase with the normal signal input and of amplitudeapproaching that of the signal input. With reference to Fig. 2, it willbe understood that this adjustment should be such that the combinationof the signal input and the adjusted auxiliary wave will produce a wavewhich takes substantially the form of the curve Ws (in the'single tonecase presented). The modulation of the carrier by this wave (W3)introduces the eiect which substantially compensates for the distortingeffect of the suppression of the sideband in filter F2, and theeffective envelope shape 0f the carrier and single`side-band energyradi-A ated from the antenna A1, is changed to a degree such that theoutput of the receiver at the distant receiving station willsubstantially correspond to the original audio-frequency input at thetransmitting station. I

In practice the degree of correspondence depends upon the extent towhich the input from transformer T3 approaches that from transformer T1;to retain normal modulation it is required only that the output oftransformer T1 be increased. VThe practical limit to the reduction ofthe distortion is determined by that distortion which arises from otherunrelated sources.

=It will therefore be understood by those skilled in the art that in theapplicants case of carrier andsingle side-band transmission withdistortion correction introduced through -a feedback circuit, thereshould be a step-up of amplitude in the audio-frequency input, incomparison with the normal carrier modulation case.

The correction introduced as described hereinabove will necessarilyresult in additional sideband components that are not present in thecase of the carrier and double side-band transmission. In other words,in order to eliminate the distortion produced at the output of thereceiver demodulator, it is necessary to transmit components which areout of phase with and will neutralize those produced in the demodulationprocess at the receiver. However, .these additional sidebandcomponentswill not linut the usefulness of the frequency band, and they need notextend beyond the band in such a way as to cause interference withadjacent channels. If, for example, the transmitted audio-frequencysignal band has a range from 0 to 10,000 cycles, the regeneration can belimited to this range by means of the audio-frequency lter F1 of Fig. 3,connected in the circuit between the rst audiofrequency amplier (AF) andthe modulator, this lter performing the dual function of limiting the fnormal and the feed-back inputs to the modulator. Since, because of thepresence of lter F1 in the circuit, no distortion component above 10,000cycles is put back into the modulator, none will be transmitted. At theoutput of the receiver these will normally be suppressed in the circuitsof the receiver, or, if not, will be tolerable because of their lowamplitude and the natural limitations of the ear.

It will be understood that, while the simple case of modulation of thecarrier by a single tone, used for illustration, would permit ofcorrecting the envelope shape of the modulated wave through the simpleadjustment of the signal input, in the practical case of thetransmission of the voice or music this would not be so, sincemodulation components result for which no cornpensation would be eiectedwithout some such arrangement as the applicants for applying theadjusted demodulated low-frequency wave.

While the invention has been disclosed as embodied in a specic form, forthe purpose of illustration, it is to be understood that with respect tosuch embodiment many changes and modications may be made within the truescope of the invention as defined in the appended claims.

What is claimed is:

l. In high-frequency transmission involving the modulation of a carrierwave by a signal wave and the suppression of one of the side-bandsresulting from such modulation, the method which consists in producingan auxiliary wave and modulating the carrier wave by said auxiliarywave, said auxiliary wave having a form such as to effect a substantialcompensation for the modulated wave envelope distortion resulting fromthe suppression of the side-band. Y

2. In high-frequency transmission involving the normal modulation of acarrier wave by an audio-frequency signal wave and the suppression ofone of the side-bands resulting from such modulation, the method whichconsists in producing an auxiliary audio-frequency wave and modulatingthe carrier wave by said auxiliary wave, said auxiliary wave having aform such as to effect a substantial compensation for the modulated waveof the side-band. l,

' 3.4In high-frequency transmission involving Y the modulation of acarrier Wave by a low-frelli) of the side-bands resulting, from suchmodulation, the carrier and the other side-band remaining'fortransmission, the method Which consists in producing an auxiliarylow-frequency Wave and modulating the carrier Wave by said auxiliaryWave, said auxiliary Wave having a form such as to eiect a substantialcompensation for the modulated Wave envelope distortion resultingV fromthe suppression of the one'side-band.

4. The method of carrier wave signaling which consists in producing ahigh-frequency Wave, modulating said Wave in accordance With the signal,suppressing one of the side-bands resulting from the modulation,producing an auxiliary lowfrequencyY Wave, and combining said auxiliaryWave with the signal for the modulation of said high-frequency Wave,said auxiliary Wave being of such form that the effect of the modulationof the high-frequency Wave by the combinatio-n lowfrequency Wave Willsubstantially compensate for the envelope distorting effect of thesuppression of the one side-band.

5.' In high-frequency transmission involving the modulation of ahigh-frequency carrier Wave by a low-frequency signal input Wave, thesuppression ofV one of the side-bands resulting from such modulation andthe transmission of the carrier and Vthe single remaining side-band, themethod which consists in deriving from the transmitted Wave ademodulated 10W-frequency Wave, effecting an adjustment of saiddemodulated lowfreq'uency (Wave, and applying the signal input Wave andthe adjusted demodulated low-frequency wavefin, combination for themodulation ofthe carrier Wave, said adjustment being suchas'substantially to compensate for the wave en- Velope distortionintroduced by the suppression of the one side-band. Y l

6. The method of high-frequency transmission which consists inmodulating a carrier Wave with the signal input Wave to produce acompositewave of carrier andptWo side-bands, supressing Aone side-band,transmitting Vthe resultant energy composed of thecarrier and oneside-band, deriving from a portioncf the transmitted energy ademodulated Ylow-frequency Wave which is a distorted representation ofthe signal input wave, effecting adjustment of the phase and amplitudeof said demodulated low-frequency wave, and

applying the signal Vinput wave and said demodulated loW-frequency wavein combination for modulation with the carrier Wave, said adjustment ofthe demodulated low-frequency Wave being such as substantially tocompensate for the modulated wave envelope distortion resulting from thesuppression of the one side-band.

'7.-The method of 'signaling over a transmission system by meansofhigh-frequency Waves which consists in vproducing at the transmittingend of the system a high-frequency carrier Wave, modulating the Vcarrierwavevvith the signal input wave to produce a composite Wave of carrierandtwo side-bands, suppressing one side-band,

transmitting theV resultant energy composed of the carrier and theremaining side-band, deriving at the transmitting end of the systemAfrom a portion of the transmitted energy a demodulated audio-frequencyWave which is a distorted :representation of the signal input wave,effecting adjustmentfof thevphase and amplitude oi saiddeting end. l

modulated audio-frequency Wave, applying thej signal input wave and theadjustedrdemodulated audio-frequency Wave in combination for modulationlwith the carrier wave at Ythe transmittingv end of' the system,transmitting the resultantV modulated Wave, after the suppression ofoneYside-band, to the receiving end of the system, and

demodulating the received Wave to produce theY iinal signal output wave,the adjustment ofthe mitting. end being such that the signal output Waveproduced at the receiving end Will represent without substantialdistortion thesignal input Wave applied tothe system at the transmit'-8. In aV transmitting circuit forming a part of a high-frequencytransmission system, means for producing. a high-frequency VcarrierWave, means for modulating saidcarrier Wave by=a low-frequency wave,means for suppressing one of the side-bands resulting from the Vaction,of said modulating means, means for producing an auxiliary low-frequencyWave of suchrform las? substantially to compensate forv the envelopedistortion resulting from Vthe action of said suppressing means, andmeans for applying said auxiliary wave to said modulating means formodulation-of said carrier wave.

9. In a signaling system, means` for producing a high-frequency carrierWave, means for modun side-bands resulting from the modulation, meanslat the transmitting endof the system for Tprothe v ducing an auxiliaryaudio-frequency wave ofV Y such form as substantially to compensate forthe envelope distortion resulting from the suppres-V sion of the oneside-band, Yand means for impressing said auxiliary Wave on. the'low-frefluency-input circuit of said modulating means.V Y

one of the side-bands produced by the modula-i tion, means fortransmitting the Wave composed of the carrier and the,V remainingside-band,

`means at the transmitting station for derivingV from a portion of thetransmitted energy an auxiliary low-frequency Wave, means forV effectingadjustment 'of said auxiliary Wave, and means for combining the signalinput vvave and the adjusted auxiliary Wave in theloW-frequen- Y cyapplication circuit of said modulator,V theV ad- Y `Vjustment ofsaid'auxiliary Wave being such'that the combined 'low-frequency, energy.applied to the modulator Will result in substantial 'compensation forthe envelope distortion introduced by the suppression of the oneside-band.r

11. In a system for-theV transmission of signalsv by the modulation V.ofa carrier Wave in accordance With the signal Wave, a modulator 'at theYtransmitting end of the system, means'forimpressing ahigh-frequencycarrier wave on said modulator, a circuit adapted to apply'loW-frequencyenergy to said modulator to modulate the carrier wave, means forsuppressing one of thelside-bands produced by the action of 'said'modulator, means for transmitting the energy` composed of the carrierand the remainingsideband, means-at the transmitting end ofV the systemfor deriving from a portion of the transmitted energy an auxiliarylow-frequency wave, means for effecting adjustment of the phase andamplitude of said auxiliary wave, and means for applying the adjusted.`auxiliary wave in combination With the signal Wave to the low-frequencyinput circuit of the modulator, said adjustment of the auxiliary wavebeing such as to produce substantial compensation for the wavedistortion resulting from the suppression of the one side-band.

12. In a system for the transmission of signals by the modulation of acarrier wave in accordance With the signal wave, a modulator at thetransmitting end of the system, means for impressing a high-frequencycarrier wave on said modulator, a circuit adapted to apply lowfrequencyenergy to said modulator to modulate the carrier Wave, means forsuppressing one of the side-bands produced by the action of saidmodulator, means for transmitting the energy composed of the carrier andthe remaining sideband, means at the transmitting end of the system forderiving from a portion of the transmitted energy an auxiliarylow-frequency Wave, means for effecting adjustment of the phase andamplitude of said auxiliary Wave, means for applying the adjustedauxiliary Wave in combination with the signal Wave to the low-frequencyinput circuit of the modulator, said adjustment of the auxiliary wavebeing such as to produce substantial compensation for the Wavedistortion resulting from the suppression of the one side-band, andmeans at the receiving end of the system for demodulating the receivedWave to produce the output signal Wave.

13. In carrier Wave signal transmission involving the suppression of oneside-band, the method of preventing the distortion normally introducedby the side-band suppression which consists in employing at thetransmitting end a carrier Wave and a modulating Wave and predistortingthe modulating Wave to a shape such as to compensate for the envelopedistortion introduced by the suppression of the one sideband.

14. In a carrier Wave signaling system, a modulator, means associatedtherewith for impressing thereon a high-frequency wave, a modui latingwave source, a circuit connecting said source and said modulator, meansfor suppressing one of the side-bands produced by the action of saidmodulator, and means associated with said connecting circuit fordistorting the modulating Wave to a shape which will substantiallycompensate for the distortion of the modulated wave envelope introducedby the suppression of the side-band.

RALPH K. POTTER.

